Track-type machines are in widespread use in construction, mining, forestry, and other similar industries. The undercarriage of such track-type machines utilizes track assemblies, rather than wheels, to provide ground-engaging propulsion. Such track assemblies may be preferred in environments where creating sufficient traction is problematic, such as those frequently found in the industries identified above. Specifically, rather than rolling across a work surface on wheels, track-type machines utilize one or more track assemblies that include an endless loop of coupled track links defining outer surfaces, which support ground-engaging track shoes, and inner surfaces that travel about one or more rotatable track-engaging elements, such as, drive sprockets, idlers, tensioners, and rollers, for example.
Typical track chain assembly designs include a track pin either fixedly or rotatably connected to a pair of chain links and a bushing rotatably positioned between the links and about the track pin. Such track chain assemblies can operate in extremely adverse environments in which track joints may be exposed to various abrasive mixtures of water, dirt, sand, rock or other mineral or chemical elements. The bearing interface between the track pin and the bushing can encounter high contact stresses which lead to galling failure. Galling is a principal failure mode for track chain assemblies and can limit the life of track chain assemblies in many applications.
Track bushings are conventionally made from carbon steel or low alloy steel that has been either direct hardened or carburized and hardened. When used in combination with this type of track bushing, any bare steel or carburized pin is limited in hardness by the hardening capacity of the carbon in steel. The surface microstructures of a direct hardened or carburized track pin can be similar to the steel bushings; this similarity is believed to limit performance.
To address galling failures, it is known to coat pins with one of several types of hard coatings, such as by using physical vapor deposition (PVD) or chemical vapor deposition (CVD). These coatings are deposited on the surface of the substrate material, and they can create a new structure that is sufficiently different from that of the bushing. These coatings can improve galling resistance. Unfortunately, deposition of these coatings is costly and adherence can be an issue.
Another approach to addressing galling failures is to increase the area of surface contact between contacting components, thereby decreasing the contact stresses and the galling propensity. This option is not desirable because increases in track pin size lead to disproportionate increases in system costs because all other related components must also follow scale and correspondingly increase in size, as well.
Nitriding is an existing heat treatment method that can create very hard thin cases on the surface of nitriding alloys. However, nitriding requires high processing temperatures (over 500° C.) which cause most steels to lose their strength. This results in having a hard case on top of a much softer substrate, which may not have the strength to support the intended service loads of the track chain assembly, including the shear and fatigue loads in large track-type machines, especially large track-type tractors.
U.S. Pat. No. 7,604,304 is entitled, “Crawler, Crawler Pin, Crawler Bush, and Crawler Manufacturing Method,” and is directed to a crawler having a crawler pin and a crawler bush having lubrication between the crawler pin and the crawler bush without lowering the strength of the crawler at the running time, thereby to prevent the creak or seizure, as might otherwise be caused at the running time. A metallic sliding member having an excellent wear resistance can be arranged in the clearance between the crawler bush and the crawler pin. The metallic sliding member can be capable of holding lubricating oil and/or a lubricant. In addition, the clearance between the core bush and the crawler pin can be narrowed, so that the resulting structure retains the imbalance load on both the crawler bush and the crawler pin.
There is a continued need in the art to provide additional solutions for a track chain assembly. For example, there is a continued need for a track pin of a track chain assembly which is not only sufficiently strong to support the service loads for which it is intended but also extremely durable to provide an extended useful life of the track chain assembly.
It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some respects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of any disclosed feature to solve any specific problem noted herein.